LOFAR observation of J1019+65 carried out on 2021-02-33. Top panel: Stokes V light curves with a cadence of 4 min at different frequencies. The shaded region represents ±1σ uncertainty. The light curve at 152-168 MHz is omitted for the sake of clarity. The top axis represents the time of observation in Modified Julian Dates (MJD). The dashed black line at 0 mJy is drawn for clarity. Bottom panel: LS periodogram of the radio light curves. The three dashed red lines represent the necessary LS power (i.e. peak height) to achieve a false-alarm probability (FAP) of 5%, 1%, and 0.3%. The grey curve represents the (negative) LS periodogram of the window function, which is a light curve with the same timestamps as the original curve, but the flux density of each data point is replaced with unity (i.e. a flat light curve). The two relevant frequencies, f₁ = 0.315 ± 0.013 h⁻¹ and f₂ = 1.247 ± 0.013 h⁻¹, are indicated by the black arrows in the LS periodogram. The 120-136 MHz (orange) peak at f₁ is consistent with the original frequency discovered by Vedantham et al. (2023), while the relevancy of the f₂ is discussed in Sect. 2.2.2. The frequency uncertainties were computed using Eq. (52) by VanderPlas (2018).